Electric motors often include mechanisms that terminate operation of the motor in response to thermal overload conditions that could result in permanent damage to the motor or associated equipment. A thermal overload, such as an excessively high winding or rotor temperature, may occur as a result of a locked rotor, a high mechanical load, a supply overvoltage, a high ambient temperature, or some combination of these conditions.
Thermal cut-outs (TCOs) are one well-known mechanism that may be used to protect an electric motor. Conventional TCOs are based on a thermally responsive element that fuses in response to a thermal overload condition, thereby interrupting the flow of electrical power to the protected apparatus. One typical approach uses a spring-loaded contact pin or lead that is held in electrical connection with an opposing contact by a fusible material such as solder. Another typical approach uses one or more springs, which are independent from a pair of electrical contacts and which urge the electrical contacts apart when a stop material melts in response to an elevated temperature. Both of these approaches are undesirable because the TCO typically includes a complex arrangement of springs and contact elements that are mounted in a housing. Thus, these approaches are inherently costly, and do not allow for the direct inspection of the TCO because the fusible material and contact conditions are not usually visible through the housing.
Conventional current fuses may also be used to protect an electric motor from thermal overload conditions. Current fuses, such as cartridge style fuses, may be serially interposed in the current path of the motor windings. Typically, the fuse is selected so that it interrupts the power supplied to the motor windings at a predetermined current level that could result in a dangerously high winding temperature. Current fuses are undesireable as a thermal overload protection mechanism because they are substantially operationally unresponsive to the actual thermal conditions within a motor, which could result in operation of the motor at a dangerously high winding temperature or a premature termination of the motor operation at a safe winding temperature. For example, a current fuse may prematurely terminate the operation of a motor in response to transient winding currents that would be insufficient to heat the thermal mass of the motor to cause a dangerously high winding temperature.